Flat-panel displays are typically constructed with an array of light emitters distributed over a flat substrate surface. With the exception of plasma televisions, emissive flat-panel displays often rely on either (i) a backlight with pixel light control provided by liquid crystals and color filters (e.g., liquid crystal displays), (ii) organic colored light emitters (e.g., organic light-emitting diode displays), or (iii) organic white-light emitters with color filters (e.g., white organic light-emitting diode displays). Importantly, all three of these flat-panel display technologies are area emitters, that is, the entire area of each pixel is filled with the light emitter or light controller. Most of these displays are active-matrix displays that rely on local circuits formed on the substrate to control the pixel. These circuits (a single transistor for liquid crystal displays and two or more transistors for organic light-emitting diode displays) require significant area on the substrate, reducing the area available for light emission. Organic light-emitting diode displays typically have a 60% fill factor, also referred to as an active light-emitting area or aperture ratio, and liquid crystal displays can have an even larger fill factor depending on the display size and resolution.
Inorganic light-emitting diodes (LEDs) are typically manufactured using a semiconductor process requiring the use of various chemicals and materials. These manufacturing methods require the use of a rigid substrate, such as a sapphire substrate or a silicon substrate, that does not melt during the high-temperature manufacturing process. After fabricating the LEDs on the rigid substrate, the wafers are often cut up to form individual LEDs that are used in displays.
Early LED applications in displays include hand-held calculators with numeric LED displays. More recently, LEDs have been integrated as backlights for displays. Integrating LEDs in larger displays, such as display panels, involves complex wiring to each individual LED in the display panel. The use of LEDs in displays, such as RGB LED displays, continues to present numerous challenges, including increased complexity, limited display format, increased manufacturing costs, and reduced manufacturing yields. For example, a display with a resolution of 1280 by 720 includes 921,600 pixels. For a RGB LED display, each pixel must typically include three LEDs (a red, a green, and a blue LED) in each pixel. Thus, the display in this example must use 2,764,800 LEDs. In some cases, all of these LEDs must be arranged in a display that measures a few inches on the diagonal. Not only must these LEDs be small, but the LEDs must be arranged in an array with the appropriate wiring and driving circuitry. Moreover, the materials used to create each color of LED vary. Arranging different color LEDs during manufacture as required for a RGB display is extremely difficult. Semiconductor chip- or die-automated assembly equipment typically uses vacuum-operated placement heads, such as vacuum grippers or pick-and-place tools, to pick up and apply devices to a substrate. It is difficult to pick up and place ultra-thin and small devices using this technology.
Further, LEDs are typically formed with terminals on different faces of the micro-LED. These vertical LEDs are challenged in electrically isolating the anode and cathode during the process of interconnection. This necessitates depositing a vertical insulator between the terminals, e.g., in the robotic assembly of the LED display. For example, if one terminal is on the bottom and one terminal is on the top, the terminals occupy the same space in the x-y plane and a robust insulator is required. The panel-level formation of vertical electrical insulation between the two terminals of the LEDs adds additional steps and layers to the display, adding increased complexity in the display application.
For these reasons, among others, it is difficult and expensive to provide high-resolution RGB LED displays for consumers. Thus, there is a need for systems and methods of manufacturing displays using LEDs that provides low-cost manufacturing, improved yield, and improved reliability for the systems.